Application of an overvoltage can lead to device destruction. A Field Effect Transistor as may for example be used as a switch in a solid relay might be rated at a maximum sustainable source-drain voltage of 50V, and would be destroyed if subjected to an overvoltage. Ways in which an overvoltage might arise are many and various. An example from an automotive application would be an event known in the art as `load-dump`.
Load-dump can occur when an auto-engine battery becomes disconnected having previously been under charge by an alternator. Upon disconnection the alternator will be subject to a sudden loss of load and since it had previously been supply a charging current immediately prior to battery disconnection field current will be high. This condition of high field current and no or substantially reduced load leads to a sudden and potential large rise in output voltage due to transformer action. An increase to 100 volts would not be uncommon, and would be sufficient to destroy low voltage (50V) devices connected to the alternator output. The condition is further worsened if significant charging current were being supplied to a lowly charged battery. Although load dump is a relatively unlikely essentially accidental event, the potential catastrophic consequences lead to its survival being part of the specification that automotive components must meet, devices within such components must therefore be protected.
In the automotive industry there is a move to replace the use of electro-mechanical components, such as for example a relay controlling the pump of an antilock braking system, with solid state components. Field Effect Transistors are commonly used as switching elements in solid state relays for example in the well known charge pump configuration. For an application having a nominal supply voltage of 12 volts (i.e. automotive battery voltage), a solid state relay might include a Field Effect Transistor switch having an absolute maximum source-drain voltage rating of 50V. Such a Field Effect Transistor clearly would not survive load dump without protection, and lack of a suitable protection circuit has heretofore ruled out the use of such devices in automotive applications. Unfortunately, the alternative solution of providing a Field Effect Transistor capable of withstanding the voltage of several hundred volts possible during load dump is not viable since such a Field Effect Transistor would be prohibitively expensive; required silicon area (a significant cost determining factor) for a device increasing in proportion to the square of the sustainable voltage.
One solution to the load dump problem is to provide a circuit for damping the supply to all components in the event of overvoltage, and to this end it is known to form the regulator diodes associated with an automotive alternator as large zener diodes which dissipate the load dump energy. Since, however, use of this solution is by no means universal, automotive component makers cannot rely on damping and continue to face the problem of load dump overvoltage survivability. Some prior circuits use a zener diode to sense load dump and develop a turn on control signal to turn on a low voltage device during load dump. However, in such circuits, this turn on signal may not be sufficiently available throughout load dump to keep the device on.